Most memories in electronic devices have transistors that connect and disconnect memory elements from the reading/writing circuits. These transistors have very high ON/OFF ratios and prevent leakage currents from passing through devices that are not selected for reading or writing. Since the transistors with the required characteristics can be fabricated only by using a semiconductor with few defects, they are fabricated on single crystalline silicon wafers. This can severely limit the design flexibility and available materials in creating memories. For example, it can be very challenging to design and fabricate high density multilayer memory using single crystalline silicon wafers.
Further, the inclusion of a transistor in the memory array decreases the density of the memory. To increase the integration density of memory devices that include transistors, the transistor needs to become smaller and smaller. However, as the transistor size decreases, it becomes increasingly difficult to satisfy the performance requirements and the fabrication costs increase prohibitively.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.